The present invention relates to coherent detection and amplitude demodulation of frequency division multiple access (FDMA) electromagnetic signals. More specifically, it relates to the field of coherent detection of suppressed amplitude modulated carriers, particularly for determining position and orientation of a moving object.
The concept of transmitting and receiving electromagnetic signals for determining position and orientation of a moving object is known in the prior art (see, for example, U.S. Pat. No. 4,314,251, U.S. Pat. No. 4,742,356, U.S. Pat. No. 4,849,692, U.S. Pat. No. 5,646,525 and the references thereof). The systems based on this concept, typically, include a plurality of transmitters generating electromagnetic fields and a magnetometer that may be any transducer, or plurality of transducers. Each of these transducers is capable of producing an output that is an analog electric signal proportional to superposition of the received electromagnetic fields. For instance, the magnetometer may receive multiplicity of signals simultaneously using frequency division multiple access (FDMA). The foregoing prior art describes various techniques, wherein the output of the detector can be converted into the moving object position and orientation relative to a reference coordinate frame associated with a source having a plurality of the transmitters.
For example FIG. 1a illustrates a practical situation in which two electromagnetic field transmitters A and B, representing any number of transmitters, generate oscillating electromagnetic fields, which are received by a magnetometer C. Each of these electromagnetic waves may be distinguished by its unique oscillating frequency. The strength of each individual field, characterized by its amplitude, is a function of coordinates, which are determined by the reference coordinate frame. The strength of the signal produced by the magnetometer depends not only on its position but also on the angle between the magnetic field and the surface of the magnetometer. The transmitted electromagnetic fields, transmitted at high frequencies, serve as carriers whose amplitude is modulated by the motion. As the magnetometer C varies its location and orientation by changing its coordinates and angles in the space over time, the amplitude of the output electric signal produced by the magnetometer also changes, thus creating a time-dependant waveform. This waveform is a superposition of all carriers, each modulated according to the location and orientation of the magnetometer. Hence, the individual waveform for each carrier is thereby determines an envelope that is a function of the magnetometer location and orientation in time. A total analog electric signal produced by the magnetometer (referred to hereinafter as a received message) is proportional to the strength of the entire electromagnetic field whose value is proportional to the superposition of the field strength of individual modulated carriers of the all transmitters.
FIG. 1b illustrates the modulation of the carriers originated from transmitters A and B each by its own envelope. FIG. 1c depicts the analog electric signal V produced by the magnetometer over time in a multi-transmitter environment of transmitters A and B. In general, the received message is proportional to a superposition of the amplitude-modulated carriers of the all transmitters within reception limit. It is relevant to note that the received message also may include some noise (not shown).
Accordingly, since the envelope modulating the carrier is a function of the magnetometer location and orientation, then demodulation of the received message for the purpose of estimation of the individual envelop corresponding to each individual carrier is a very important task.
Commonly, a digitally implemented phased-locked loop (PLL), in a digital signal processor (DSP), is assigned to the task of demodulating the received message. PLL in general, is fairly known in the prior art (see, for example, Jacob Klapper and John T. Frankle, Phase-Locked and Frequency Feedback Systems, Academic Press, New York, 1972; Ch. 8), however these techniques were employed usually for carrier recovery rather than for processing the received message and envelope demodulation.
There is, accordingly, a need in the art to provide a method and a system for envelope extraction based on the PLL technique. The quality of the envelope estimate depends on the ability to overcome all sources of inaccuracies such as coupling between the individual carriers and harmonic distortions while preserving a spectral bandwidth covering the spectrum of the modulating signal. In addition, the demodulation process should be performed in real time and introduce only a short processing delay. This is relatively complicated computation task, which adversely effects the DSP performance and may interfere with real-time requirements. An application of the PLL technique in DSP may allow coping with these problems.
One has to bear in mind that the modulating signal may take either positive or negative value. Thus recovering the sign of the envelope is essential for unambiguous tracking. A specific design of PLL, namely tanlock may be used when the sign of the envelope is of importance as in the case for magnetic trackers. While implementation of tanlock design in PLL technique is known in the prior art (see, for example, xe2x80x9cPerformance Analysis of Digital Tanlock Loopxe2x80x9d, Jae Cohn Lee and Chong Kwan Un, IEEE Trans. Comm. 30:2398-2411; xe2x80x9cNoise Analysis of a Digital Tanlock Loopxe2x80x9d, Carlos A. Pomalza-Raez, IEEE Trans. AES 24:713-718; U.S. Pat. No. 4,068,210, U.S. Pat. No. 4,782,532, U.S. Pat. No. 4,872,207), it was not applied therein for envelope extraction.
Moreover, an ordinary PLL, including tanlock-type PLL, are both designed to respond as quickly as the signal-to-noise ratio permits. However, this is a drawback in the problem where a signed enveloped is needed. Rather, a slow response is desired such that the phase of the voltage-controlled oscillator (VCO) of a phase locked loop would preserve its phase when the sign of the envelope flips.
It should be noted that during the process of envelope extraction, it is necessary to synthesize sine and cosine trigonometric functions. One common approach for the synthesis of the trigonometric functions is to build a lookup table. This method can be further enhanced by interpolation between table entries (see, for example, U.S. Pat. No. 4,905,177). Alternatively, sine and cosine waveforms can be synthesized using a real-time solution of a difference equation (see, for example, U.S. Pat. No. 4,888,719). Several enhancements to these methods were disclosed (see, for example, U.S. Pat. No. 5,113,361, U.S. Pat. No. 4,761,751, U.S. Pat. No. 5,631,586). Despite the apparent superiority of the difference equation method, due to the finite precision of the computer, implementation of difference equation solution in DSP may produce an accumulating error. The error may lead to both phase and amplitude drift (see, for example, U.S. Pat. No. 4,285,044) affecting the accuracy of the envelope calculation, and thus a control mechanism is required.
There is, accordingly, a need in the art to provide an improved method and system for coherent detection, and demodulation of a FDMA signal comprising a superposition of different carrier amplitude modulated signals. The improved method and system utilize a phase locked loop technique for the envelope extraction and might cope with the drawbacks of the hitherto known usage of the PLL technique. There is a further need in the art to provide a novel technique for generation of the sine and cosine trigonometric functions within a PLL.
According to the invention, a magnetometer in a multi-transmitter environment receives a signal originated from several transmitters and produces an s analog electric signal, which is the received message, that is proportional to superposition of amplitude-modulated carriers of the all transmitters. Further, this analog electric signal is digitized and all signal processing is performed digitally.
In accordance with a preferred embodiment of the invention, an array of special digital phase-locked loops (PLLs) is implemented in a DSP for processing the received message after it is digitized and extracting the envelopes corresponding to each carrier. Each loop is tuned and locked on a single carrier. Thus, each member of the loop-array produces an estimate of the envelope of a single carrier.
In accordance with the invention, the PLL is implemented in a DSP and used for coherent envelope extraction is deliberately made to slowly respond to error signals. This mechanism allows the loop to preserve its phase while the amplitude value of the envelope flips sign.
In accordance with another aspect of the invention, a down-decimation technique is employed in order to reduce the computational load. This is achieved by applying a finite impulse-response filter (FIR) only once per each frame, thereby only a single multiplication of the frame by the vector of filter""s taps is employed.
In accordance with yet another aspect of the invention, a digital generation of sine and cosine sampled waveforms is employed inside a PLL by using a real-time solution of a difference equation. Implementation of the sine and cosine generation inside a PLL provides an automatic control of the sine and cosine phase.
In accordance with a broad aspect of the invention there is provided a method for coherent detection and demodulation of a FDMA signal being a superposition of amplitude modulated carriers, the method comprising the steps of:
(i) receiving said FDMA signal by a system for detection and demodulation of the signal; and
(ii) processing the signal by using a PLL technique and extracting an envelope of at least one carrier.
In accordance with another broad aspect of the invention there is provided a system for coherent detection and demodulation of a FDMA signal being a superposition of amplitude modulated carriers of said transmitters, the system comprising a digital coherent detector for receiving said FDMA signal, processing the signal and extracting a signed envelope of at least one amplitude modulated carrier, the detector comprising:
(i) a transducer for transforming said FDMA signal into analog electric signal,
(ii) a front-end electronics for receiving said analog electric signal and providing pre-amplification and pre-filtering of the input signal,
(iii) an analog to digital converter (ADC) for translating the electric signal coming after said front-end electronics into digitized signal data sampled at fixed ADC sampling intervals, and
(iv) a digital signal processor (DSP) for processing the digitized signal data and providing a real-time estimate of the signed envelope of at least one amplitude modulated carrier,
the detector synchronized by a synchronization signal delivering initial phase information to the DSP for estimation of the envelope sign.